The proposed research is a genetic and biochemical investigation of the molecular mechanisms regulating gene expression by the bacterial virus lambda and its host E.coli. One of the major goals of this work is to analyze the complex interactions among phage and bacterial proteins which regulate the choice between lytic and lysogenic growth by the phage; in particular, those factors which control phage repressor synthesis and lysogenization. A central determinant in the lysis-lysogeny decision appears to be the level of activity of the phage cII protein. This level is itself controlled by activity of a bacterial protein coded by the hfl gene, which appears to be a protease that degrades cII protein. We have cloned the hfl gene that has been most intensively studied and propose to produce defined mutations in this gene and to introduce them into the bacterial genome in order to determine the role of Hfl for E.coli. We have also isolated fusion between the promoter for hgl and beta-galactosidase, which will allow us to determine whether any physiological factors affect expression of the hfl gene. In addition, we have begun studying a second hfl gene and have preliminary evidence that it controls proteolysis. Further studies of this gene (its map position, phenotypes, etc.) are proposed.